Electrophoretic printing medium



Oct. 7, 1969 0. J. J. LENNON ETAL I 1,

ELECTROPHORETIC PRINTING MEDIUM Filed Aug. 2, 1965 INVENTORS DONALD J. J. LENNON H A RVEY G. SCHLEIFSTEIN KM. r M

- f ATTORNEYS United States Patent f 3,471,387 ELECTROPHORETIC PRINTING MEDIUM Donald Joseph John Lennon, Acton, and Harvey G.

Schleifstein, Brookline, Mass., assignors to The Carters Ink Company, Cambridge, Mass., a corporation of Massachusetts Filed Aug. 2, 1965, Ser. No. 476,424 Int. Cl. B01k 5/00 US. Cl. 204-180 4 Claims ABSTRACT OF THE DISCLOSURE Electrophoretic printing as described by Oster, United States Patent No. 3,145,156, is carried out using a source sheet in which the color material is undispersed and a liquid medium in which it is a nonsolvent. The application of an electrical field causes the color material to disperse and transfer from the source sheet to a recipient sheet.

This invention relates to electrophoretic printing and in particular to the prevention of unwanted migration of color in the printing process.

Electrophoretic printing is described in Oster, United States Patent No. 3,145,156, and consists generally in causing selective electrophoretic transfer of charged color particles from a source sheet to a contiguous recipient sheet. In carrying out the process, a source sheet containing charged or ionizable color particles and wet with an appropriate liquid, is placed in face to face contact with a similarly moistened recipient sheet. The two sheets are then placed between electrodes and an electrical field is applied selectively to cause the electrophoretic transfer of color particles from the source sheet to the recipient sheet, the shape of the field controlling the shape of the pattern printed.

In practice, it has been found that when the moistened source sheet and recipient sheet are placed together, there is a natural tendency for the particles of color material to migrate by diffusion into the recipient sheet. Similarly, if particles of opposite charge are present in the same sheet, they may tend to agglomerate into uncharged masses.

As described in the copending application of Donald J. J. Lennon, Ser. No. 338,245, filed Jan. 16, 1964 now US. Patent 3,372,102, a degree of control over unwant d migration of color particles can be provided by causing the color particles to be adsorbed by an adsorbent material, which serves to hold them until they are dislodged by the electrophoretic field. This technique, however, has the disadvantage of requiring the presence of otherwise unnecessary ingredients which themselves may become dislodged in the course of the electrophoretic process to cause fouling of the printing electrodes. Moreover, where the source sheet is initially dry and moistened just preliminary to the printing operation, the presence of adsorbent material frequently results in its accumulation in the moistening liquid, which must then be renewed frequently.

The present invention is directed primarily to electrophoretic printing processes in which a moistened source sheet containing color material and a moistened recipient sheet to be printed upon are brought together, placed between electrodes and then print activated by the application of an electric field. It has now been found that for a given color material present in the source sheet, a moistening liquid may be compounded in such a manner that the color material is not soluble sufficiently to migrate until the electrical field is applied. Under such conditions it appears that the energy of the field becomes 3,471,387 Patented Oct. 7, 1969 a factor in placing the color material in condition for electrophoretic migration.

In general, it has been found that a dye stuff may be brought into contact with a mixture of a solvent and a non-solvent in proportions such that the dye stuflf is not soluble and will not diffuse or migrate until an electrical field is applied.

In carrying out the invention a source sheet, convehiently a sheet of adsorbent paper or textile fabric, is impregnated with a solution of dye stuff in a solvent therefor and is then dried. Prior to use in the electrophoretic printing process the source sheet is remoistened, this time with a mixture of solvent and non-solvent liquids, so proportioned that the dye stuff remains essentially out of solution. The remoistened source sheet is then combined with a similarly moistened recipient she t and placed between the electrodes, which are then energized from an appropriate voltage source.

In selecting the dye and moistening liquid, it is desirable that the non-solvent component of the moistening liquid be the least volatile, so that as the recipient sheet dries the dye does not go into solution and bleed.

The dye stuffs that are presently preferred are those classified as alcohol soluble, and the preferred remoistening liquid consists typically of an alcohol or a ketone as the solvent component and water as the non-solvent component.

Since the dye stuff is essentially insoluble in water, the alcohol or ketone is preferably one that is more volatile than water, so that bleeding does not occur as the recipient sheet dries.

If on the other hand, water soluble dyes were employ d in conjunction with remoistening liquids consisting of water and a more volatile organic compound, drying would bring about a condition of increased dye solubility because of the escape of the organic compound, leaving water as the residual liquid. This could, of course, be avoided by employing organic non-solvents which are less volatile than water, but the result would be a much more slowly drying system.

Oil soluble dyes have for the same reason as yet not been found to olfer great promise, generally because of the low volatility, and insulating nature, of most organic systems. It is however contemplated that systems based upon the presence of volatile hydrocarbons, a less volatile non-solvent, and, if necessary, a common solvent for the two other liquid components to cause them to mix would be useful.

The following example sets forth in detail a preferred embodiment of this invention.

The source sheet was prepared by impregnating a standard nylon typewriter ribbon .004 inch thick with a solution containing 30% by weight of Iosol dye in methyl alcohol. In order to provide an intense black color, a mixture of the dyes were employed as follows:

Parts by weight Iosol black 1 5 Iosol red Iosol yellow 3 1 National Aniline, Solvent black 13.

2 National Aniline, Solvent red 68. 3 National Aniline, Solvent yellow 42.

the anode characters was formed, at rates corresponding to 10,000 lines per minute.

Other suitable remoistening liquids are a mixture of water and methanol, and water and amyl alcohol.

The printing apparatus is shown schematically in cross section in the accompanying drawing.

The source sheet and recipient sheet 12, both moistened as described above, are placed between a base electrode plate 14, and a printing electrode 16 having exposed conductive characters 18. The printing electrode consists of a brass type slug 20 having raised portions 22 defining the characters 18, embedded in an epoxy resin casing 24 with the character faces exposed. Conveniently the printing electrode is formed by putting the type slug 20 in the epoxy casing 24 and then grinding away the printing face until the characters 18 are exposed.

A voltage source 26 connects to the electrodes 14 and 16 by leads 28 and 30, through a switching device 32.

The switching device 32 and source 26 may together comprise a square wave generator, and means (not shown) may also be provided for advancing the sheets 10 and 12 for multiple high speed printing.

The switching means 32 and source 26 may also include means for reversing the polarity of the electrodes, in order to print from an upper source sheet and a lower recipient sheet, or to print with a dye having the opposite sign of charge. In this connection the apparatus may be used for printing in either of two colors, by providing a source sheet having both an anionic and a cationic dye, and reversing the polarity to print with the other dye and change the color.

Although this invention has been described with reference to its preferred embodiment, it is contemplated that those skilled in the art and familiar with the principles herein disclosed will readily make obvious modifications without departing from the scope of this invention. On will readily select other dyes, and other solvent systems for them and determine by routine experimentation their relative utilities in the practice of this invention. It is in this connection not contemplated that the invention is limited to particular dyes, and particular solvent systems, since others may readily be selected by those skilled in the art on the basis of this disclosure.

For example, for any given dye there may be readily prepared a mixture of solvent and non-solvent in such proportion that the dye is not soluble in the mixture. The application of such a mixture to a source sheet containing the dye will provide readily for the determination of th suitability of the mixture on the basis of which it may be Particularly suitable with Spirit Nigrosine SSB (Solvent black 5, C1 50410) 2 E.g., Fuchsine N., Du Pont Cl 42510.

accepted or rejected in the same manner as those in the mechanical arts select materials by reference to their hardness, strength, density, or other established criteria.

Having thus disclosed our invention and described in detail its preferred embodiment, we claim and desire to secure by Letters Patent:

1. The method of electrophoretic printing comprising providing a source sheet containing undispersed color material capable of electrophoretic transfer and a recipient sheet in surface to surface contact, moistening said source sheet with a liquid which is a non-solvent for said color material, and establishing an electrical field effective to cause electrophoretic migration across said sheets to transfer color material from said source sheet to said recipient sheet.

2. The method of electrophoretic printing comprising providing a source sheet containing undispersed color material capable of electrophoretic transfer and a recipient sheet in surface to surface contact, moistening said source sheet with a liquid which is a mixture comprising a solvent for said color material and a non-solvent for said color material, said liquid being compounded in such a manner that the color material is not soluble sufficiently to migrate until an electrical field is applied, and establishing an electrical field effective to cause electrophoretic migration across said sheets to transfer color material from said source sheet to said recipient sheet.

3. Electrophoretic printing sheet comprising in combination, a source sheet containing undispersed color material capable of electrophoretic transfer and a liquid mixture comprising a solvent for said color material and a non-solvent for said color material, said solvent being more volatile than said non-solvent, said liquid being compounded in such a manner that the color material is not soluble sufficiently to migrate until an electrical field is applied.

4. The printing sheet defined by claim 3 wherein the color material is an alcohol soluble dye stuff, the solvent is selected from the group consisting of alcohols and ketones and the non-solvent is water.

JOHN H. MACK, Primary Examiner E. ZAGARELLA, JR., Assistant Examiner US. Cl. X.R. 

